How do I fix this triangulation issue?
You need to fill the faces with quads or triangles, or remove the smooth shading on the nogons. 

You can try (CtrlT) then (AltJ), to fill it with quads, as a quick fix.

A better method would be to repair your modeling.




Delete the vertices on the ends of your N-gons:

Select one of the edge loops at the bottom or top.

Extrude the vertices out(EX y or Z)

scale them to zero along the axis that you extruded them out(SX y or Z0).

Do the same for the bottom vertices.